This invention relates to a novel silicon-based conductive material in which the silicon is made to contain various elements in relatively large quantities, and more particularly to a novel silicon-based conductive material that allows substrates, chips, and the like to be made smaller and increases productivity by implanting phosphorus, boron, aluminum, or the like in a silicon substrate with:an ion beam in a pattern so that the required areas become conductive, wherein this silicon-based conductive material can be worked into a sheet or rod and utilized in connector terminals, contacts, and so forth, or can be made into fines and dispersed in a resin or glass to produce a conductive sheet material, for example, and is therefore suitable for any application that requires electrical conductivity.
Electrically conductive materials need to have a variety of characteristics. For example, in conductive wire applications, such as the wiring of semiconductor devices or various types of electronic and electrical devices, the electrical resistance must be low, corrosion resistance and mechanical properties need to be excellent, and connection must be easy. Copper and aluminum, as well as alloys such as copper alloys and aluminum alloys, have often been used for this purpose.
Depending on the package material, a variety of alloys have been employed for the lead frame materials of conductive sheets and strips, typified by semiconductor lead frame materials. Among those that have been used are Fexe2x80x94Ni, Cuxe2x80x94Fe, Cuxe2x80x94Sn, and Cuxe2x80x94Zr systems.
Various alloy materials, such as those based on copper, carbon, silver, gold, or a platinum family metal, are used as contact point materials, which need to be conductive and resistant to arcing and wear.
Methods that have been- adopted for manufacturing conductive plastics, which are produced by imparting conductivity to a plastic (which is an insulator) and which are utilized for antistatic purposes, involve admixing carbon black, carbon fiber, or a metal powder or fiber into a resin.
The various electronic and electrical devices of today are more compact and lightweight because of higher packaging density, accomplished by putting resistors, capacitors, diodes, and transistors on a chip, but advances in chip-in-chip technology are making even higher packaging density possible, with the density of go-called printed wiring being increased (for example, copper foil of less than 20 xcexcm is being used), and wire bonding is also becoming ultrafine.
Multilayer thin film circuits have been proposed in an effort to further advance chip-in-chip technology. A conduction thin film with a width of no more than 3 xcexcm and a thickness of no more than 0.1 xcexcm is formed, and three-dimensional wiring is achieved by through-holes in the insulating films between layers. Aluminum films are used for these conduction thin films, and copper films are increasingly being employed in CPU applications.
Metals and alloys are used for conductive wires, such as the wiring of semiconductor devices or various: types-of electronic-and electrical devices, and in addition to their use as multilayer thin film circuits, these materials. have also been used as sheets, strips, and wires so as to allow connection to a mounting substrate or semiconductor chip, but this has been an impediment to increasing fineness and density and obtaining a smaller and lighter product.
The inventors came to the conclusion that if the required conductivity could be ensured without using conductive wires or the like on the chip, as is the case with an ordinary silicon substrate or other such semiconductor device substrate then devices could be made thinner and smaller, a reduction in the number of parts could be achieved, and various devices could be packaged more compactly on a single substrate.
The electrical resistivity p of a semiconductor is generally held to be 10xe2x88x922 to 109 (xcexa9xc2x7m). Silicon is a semiconductor with a diamond structure, and its electrical resistivity p is 2.3xc3x97105 (xcexa9xc2x7m), but it can be made into a p- or n-type semiconductor by adding boron or phosphorus as impurities. Silicon can also be used over a wide range of temperatures, and as a semiconductor it allows current to be controlled, and is therefore widely used in devices today. This pn control is achieved by doping the silicon with only a tiny amount of impurities (about {fraction (1/10,000)}th), and it is known that pn control is impossible with doping in larger quantities.
Meanwhile, it is known that silicon is metallized when a large quantity of impurities is introduced. An article titled xe2x80x9cLow-Temperature Magnetoresistance of a Disordered Metalxe2x80x9d by T. F. Rosembaum and R. F. Milligan (Physical Review Letters, pp. 1758-1761, dated Dec. 14, 1981) reports on the magnetoresistance of metallic Si-P at 100 mK. It is reported that the critical density at a temperature of 3 mK is nc=3.74xc3x971018 cmxe2x88x923, and the electrical resistivity p is 2xc3x9710xe2x88x922 (xcexa9xc2x7m).
Also, salicide technology, in which a silicide layer is formed on the gate electrodes and diffusion layer surfaces of the source and drain, has been developed in an effort to lower resistance within an element in the manufacture of a MOSFET, and materials that have been studied include TiSi2, NiSi, and CoSi2.
None of the above-mentioned silicon semiconductors, metallic Si-P, or suicides had an electrical resistivity p that was any better than that of semiconductors (10xe2x88x922 (xcexa9xc2x7m)), and could not be used as conductors for xe2x80x9ccarrying current.xe2x80x9d
As mentioned above, however, system integration is possible if polycrystalline Si-TFT can be formed on a single glass substrate and various devices such as microprocessors formed on the surrounding substrate, but it is believed that packaging would be easier if conduction could be ensured with the very material used to form a film on the glass substrate, and particularly a silicon-based material other than a metal.
It is an object of the present invention to provide a silicon-based conductive material that is based primarily on semiconductor silicon, can be easily manufactured, is easy to handle, has an electrical resistivity at normal temperature of 10xe2x88x923 (xcexa9xc2x7m) or less, which could not be achieved up to now, and furthermore attains the electrical resistivity that is commonly found in semiconductors (10xe2x88x926 (xcexa9xc2x7m) or less), and can be provided in the required pattern within a semiconductor silicon substrate, or made into a substrate, rod, or wire, or can be made into fines and dispersed in a resin or glass to produce a conductive sheet material, and is therefore suitable for any application that requires electrical conductivity.
The inventors conducted various studies aimed at finding a material based primarily on semiconductor silicon and with which an electrical resistivity at normal temperature of 10xe2x88x923 (xcexa9xc2x7m) or less, or even 10xe2x88x926 (xcexa9xc2x7m) or less, could be attained, which was impossible in the past. In the course of this investigation, they turned their attention to the conventional belief that if various dopants were added to silicon alone, the energy state density decreased and the Seebeck coefficient would also go down steadily as the added amount was increased, that is, the belief that the decreases in energy state density and the Seebeck coefficient were a result of an increase in the band width of the impurity level in the band gap along with an increase in the carrier concentration (A. F. Joffe: Semiconductor Thermoelements and Thermoelectric Cooling, Infosearch, London, 1957).
In view of this, the inventors came to the conclusion that if the carriers are at a certain specific concentration, there is an electron correlation or hole correlation at work between the electrons or holes that are the carriers, and conversely that the energy state density of the carriers is higher through the segregation of the carriers in the semiconductor, that is to say that Anderson segregation is occurring (P. W. Anderson, Phys. Rev., 102 (1958), 1008). In other words, even though the carrier concentration increased up to a specific density, electrical resistance continued to decrease, but the inventors thought that the Seebeck coefficient might increase sharply at a certain carrier concentration, which would result in a marked increase in the performance index.
The inventors learned that adding Group 3 or 5 elements to silicon alone causes the Seebeck coefficient to be equivalent or higher on the basis of the above assumption, and far higher at a specific carrier concenrtration, compared to the Sixe2x80x94Ge and Fexe2x80x94Si systems known in the past, and confirmed the validity of the above assumption through various experiments, without losing the fundamental advantages had by silicon alone. They also discovered a favorable composition and texture for a thermoelectric conversion material.
On the basis of their findings about novel silicon-based conductive materials based on the above-mentioned assumptions, the inventors produced silicon-based materials by adding various elements for producing a p-type semiconductor and elements for producing an n-type semiconductor (elements other than Group 3 or 5 elements) to silicon, and examined how the doping amounts thereof are related to carrier concentration and electrical resistance. As a result they learned that electrical resistance can be lowered by adjusting the carrier concentration by the addition of the above-mentioned various dopants. For instance, if zinc, boron, phosphorus, aluminum, gallium, neodymium, yttrium, or the like is contained in an amount of at least 0.001 at %, the result will be a p-type semiconductor with an electrical resistivity p of 1xc3x9710xe2x88x923 (xcexa9xc2x7m) or less, and if the amount contained is at least 1 at %, the conductive material will have an electrical resistivity of 1xc3x9710xe2x88x926 (xcexa9xc2x7m) or less, and in a favorable case 1xc3x9710xe2x88x927 (xcexa9xc2x7m) or less.
The inventors further learned that it is particularly effective for the dopants to be one or more members of the group consisting of dopants A (Be, Mg, Ca, Sr, Ba, Zn, Cd, Hg, B, Al, Ga, In, Tl) and transition metal elements M1 (Y, MO, Zr) which are known as dopants for making silicon into a p-type semiconductor, and dopants B (N, P, As, Sb, Bi, O, S, Se, Te), transition metal elements M2 (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, Au; where Fe accounts for 10 at % or less) and rare earth elements RE (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu) which are known as dopants for making silicon into a n-type semiconductor. These elements are especially effective when they are added singly or in combination. Further, a similar conductive material will be obtained even with elements other than those listed above.
The inventors also found that the previously discovered favorable composition and texture for a thermoelectric conversion material allow the product to exhibit good characteristics as a conductive material as well. Further investigation aimed at lowering the electrical resistivity xcfx81 revealed that there are no elements unsuitable as dopants to silicon, the optimal added amount varies with the selected element, and this range tends to vary when two or more dopants are used; that a conductive material with any electrical resistivity p or around the 10xe2x88x927 (xcexa9xc2x7m) can be obtained, regardless of the texture state, by manufacturing the conductive material by a method and within a range that does not result in metallization or compounding at the added amount corresponding to the selected element; and that conductive lines can be freely provided toga silicon substrate, particularly using ion implantation. These discoveries led to the present invention.
Meanwhile, the inventors examined various doping methods in which silicon is doped with a variety of elements, and as much as possible the dopants are added in amounts that will result in the specified component proportions in order to obtain a silicon-based conductive material with low resistivity and in which the carrier concentration is 1017 to 1021 (M/m3). As a result, they found that compositional deviation can be minimized by producing a compound of silicon and dopants ahead of time, and adding to silicon alone and melting in the form of a compound, so that the melting point of the added compound is closer to the melting point of silicon.
Specifically, they learned that the carrier concentration can be controlled more uniformly and more precisely by using a silicon-based compound such as Al4Si, B4Si, Mg2Si, Ba2Si, SiP, SiO2, SiS2, or Si3N4 in the doping of Group 3 elements such as B, Al, Ga, In, and Tl and Group 5 elements such as N, P, As, Sb, and Bi, or Group 2 elements such as Be, Mg, Ca, Sr, and Ba, Group 2B elements such as Zn, Cd, and Hg, and Group 6 elements such as O, S, Se, Te, and Po, for example, as the dopants used to control the carrier concentration in the silicon semiconductor. Further investigation was conducted to find whether a silicon raw material with even lower purity could be used, and as a result even a raw material with a purity of 3N could be used satisfactorily.
Furthermore, the inventors conducted various investigations aimed at further lowering the resistivity of a silicon-based conductive material, which led to the hypothesis that the above-mentioned problems could be solved by creating a metal conduction grain boundary phase that is discontinuous with the very fine semiconductor grain phase in the semiconductor bulk. The term xe2x80x9cmetal conduction grain boundary phasexe2x80x9d as used here refers to a metal phase or semi-metal phase that undergoes a Mott transition and has a carrier concentration of at least 1018 (M/m3).
In view of this, the inventors realized that the semiconductor phase and the metal conduction grain boundary phase are indistinct with a powder metallurgy process because dopants are present in a large quantity in the semiconductor crystal grains after sintering and that the electrical resistivity of the semiconductor phase decreases. They therefore conducted an investigation aimed at allowing the semiconductor grain phase to be separated from the metal conduction grain boundary phase by arc melting.
In order to lower the thermal conductivity of a silicon semiconductor, the inventors added Group 2 and 3 elements to silicon alone with a p-type semiconductor, and added Group 5 and 6 elements to silicon alone with an n-type semiconductor, after which each was arc melted in an argon atmosphere, immediately after which each was quenched by being held down from above with a chiller, for example, which produced thermoelectric conversion materials having fine crystal grains with an average diameter of 0.1 to 5 xcexcm. The electrical resistance of these materials was examined, which revealed that there was almost no precipitation of dopants at the grain boundary in silicon semiconductor bulk when the total amount of the various elements added to silicon alone was less than 0.001 at %, so the electrical resistivity was high, but when the amount exceeded 0.001 at %, some of the dopants began to precipitate at the grain boundary, and at 1.0 at % this precipitation effect markedly lowered electrical resistivity.
Specifically, the present invention is a silicon-based conductive material, in which the silicon contains at least one other element in an amount of 0.1 to 25 at %, or over 25 at % depending on the type of selected element or when two or more are used, and similarly contains preferably 0.5 to 20 at % depending on the type of element selected from the group of dopants for producing the above-mentioned p- or n-type semiconductors or when two or more elements are used, and has an electrical resistivity xcfx81 of 1xc3x9710xe2x88x923 (xcexa9xc2x7m) or less, and in a favorable case 1xc3x9710xe2x88x926 (xcexa9xc2x7m) or less.
The silicon-based conductive material of the present invention, when quenched by any of various methods after melting, has good conduction characteristics and is composed of a semiconductor grain phase and a conductor grain phase of a metal or semi-metal dispersed in bulk.
Also, the silicon-based conductive material of the present invention has good conduction characteristics when atoms of another element are substantially dispersed among the silicon lattices by ion implantation or another such method.